Most industrial processes for the alkylation of aromatic hydrocarbons employ a Friedel-Crafts type of catalyst such as aluminum chloride, boron trifluoride, hydrofluoric acid, liquid and solid phosphoric acid, sulfuric acid and the like. These materials are highly corrosive to process equipment, cause many operational problems, and are often difficult to dispose of in an environmentally acceptable manner.
It has long been a goal of research to find noncorrosive, solid catalysts to replace the Friedel-Crafts catalysts. To this end, various types of zeolite catalysts have been proposed.
U.S. Pat. No. 4,891,458 to Innes et al. discloses a process for the alkylation of an aromatic hydrocarbon which comprises contacting the aromatic hydrocarbon with a C.sub.2 l to C.sub.4 olefin alkylating agent, under at least partial liquid phase conditions, and in the presence of a catalyst comprising zeolite beta.
U.S. Pat. No. 4,954,325 to Rubin et al. discloses a synthetic porous crystalline material with X-ray diffraction lines similar to SSZ-25. This patent discloses 16 different utilities for this material. Included among these is the suggestion that this material effectively catalyzes the alkylation of aromatic hydrocarbons with gaseous olefins to provide short chain alkyl aromatics, such as cumene (See Col 6, lines 49-56.) Disclosed reaction condition range from 10.degree. C. to 125.degree. C. (50.degree. F. to 252.degree. F.).
Despite the available literature on zeolite catalysis and the recognized advantages of using a noncorrosive catalyst, zeolites are not always the preferred catalysts for use in industrial alkylation processes. Under commercially realistic conditions, many of the zeolite catalysts described in the literature have tended to deactivate rapidly due to fouling, produced the desired product in a lower yield than the competing Friedel-Crafts catalyst, or made a product which failed to meet established purity specifications.
Although some of the zeolites described in the art may be useful for commercial alkylation processes, the art is currently unpredictable with regard to whether a zeolite catalyst will perform effectively for production of ethyl benzene and/or cumene.
On a related front, the requirement that lead be phased out of gasoline and the introduction of premium unleaded gasoline has created a strong demand for increased gasoline octane numbers. Conventional approaches such as increasing operating severity in reformers and fluid catalytic cracking units, or using octane catalysts or additives in the fluid catalytic cracking (FCC) result in losses of gasoline yield. In addition, these approaches often increase the fuel gas yields in a refinery which may sometimes cause a reduction in refinery throughput and profitability. Also, these approaches often result in increased benzene content in the gasoline.
Typical gasoline contains 1 to 5 liquid volume percent benzene, a chemical which has a high octane blending value, but is considered environmentally hazardous. The State of California, for example, has included benzene in its toxic chemicals list, and the State of California Air Resources Board and the United States Environmental Protection Agency are considering regulations to limit the amount of benzene which may be present in gasoline. It is anticipated that a pending change in the Clean Air Act will also limit the allowable percent of total. It is therefore highly desirable to remove benzene from gasoline blend stocks.
Physically separating benzene from gasoline by solvent extraction has the undesirable effect of decreasing both the octane rating and the volume of gasoline.
As an alternative, benzene in gasoline blend stocks may be hydrogenated to a nonaromatic compound. This approach is also undesirable, however, because it requires a relatively high pressure operation and consumes hydrogen which is usually expensive in a refinery. Hydrogenation of benzene also reduces the octane rating of the gasoline.
To overcome these disadvantages, it has been found that benzene may be alkylated with resulting actual improvements in both the octane and the volume of gasoline produced. Co-pending U.S. patent application Ser. No. 64,121, filed Oct. 28, 1988, discloses reacting a refinery stream with an olefin-containing stream in a distillation column reactor in the presence of an alkylation catalyst (beta zeolite) to thereby alkylate light aromatics, particularly benzene.
The chemical reactions involving alkylation of aromatics with olefins are well known. For example, U.S. Pat. No. 2,860,173 discloses the use of solid phosphoric acid (SPA) as a catalyst for the alkylation of benzene with propylene to produce cumene. U.S. Pat. No. 4,347,393 discloses the use of Freidel Crafts catalyst, especially aluminum chloride for this reaction. More recently, certain rare earth modified zeolites and Mobil's HZSM-5 zeolite catalyst have been used to carry out this reaction. Examples may be found in the Journal Catalysis Volume 109, pages 212-216 (1988). Similarly, the alkylation of benzene with ethylene to produce ethylbenzene is a well known commercial process. The Mobil/Badger ethylbenzene process produces high purity ethylbenzene in vapor phase with a multiple-bed reactor and a series of distillation columns. A description of the process using a dilute ethylene stream may be found in the Oil and Gas Journal, Volume 7, pages 58-61 (1977).
A paper entitled "Alkylation of FCC Off Gas Olefins with Aromatics Via Catalytic Distillation", by I. E. Partin was presented at the National Petroleum Refineries Association Meeting, Mar. 22, 1988. This paper discloses a catalytic distillation process which uses the refiners light olefin gases such as ethylene and propylene, present in FCC and coker unit tail gas to alkylate light aromatics such as benzene and toluene, present in reformate. In the process as taught in this paper full range reformate is charged to the lower distillation section and the total FCC off gas stream is charged beneath the catalyst section. The solid proprietary catalyst is secured within supports which form bundles for installation in the distillation tower. As olefins and aromatics proceed into the catalyst section and react, the heavier alkylated aromatics move down out into the lower fractionation section and out the bottom of the tower with the bulk of the reformate. Light components, including light gases, proceed up the reactor and are stripped out in the upper distillation section. Part of the unreacted benzene is recycled back to the tower to increase benzene conversion. Noncondensible gases go to fuel and light liquid is circulated back to the refinery gas plants or to gasoline blending.
It is important to distinguish that while catalytic aromatic alkylation is known, it is subject to the unexpected and unpredictable vagaries of catalytic processes. For example, in U.S. Pat. No. 3,527,823 (Jones) there is disclosed the reaction of benzene and propylene over phosphoric acid catalyst in a nondistilling upflow reactor to produce cumene. While the benzene-propylene reaction was successful, the Jones process was not applicable to the reaction of benzene and ethylene (Column 13, line 36 of Jones). Poor yields of ethyl benzene were obtained. However, increasing ethylene purity increased the conversion of ethylene (Column 13, line 10 of Jones) although the yield of ethyl benzene was still not satisfactory. In another U.S. Pat. No. 3,437,705, Jones discloses the alkylation of an aromatic compound in a process operating with an aromatic to olefin mole ratio of from 2:1 to 30:1. The process is characterized by the presence of an unreacted vapor diluent, such as propane, in the reaction zone. The total alkylation effluent is passed to a flash distillation zone where the unreacted diluent is separated. The process is purportedly applicable to a variety of reactions using feedstocks containing unreactive vapor diluents.
Another point to be kept in mind is that operation in a nondistilling reactor system is significantly different from operation in the alkylation zone of a distillation reactor. In the former, in the alkylation zone the reactants and the products of reaction remain in contact with the catalyst throughout the process. As a result, the products can be adsorbed by the catalyst and thereby shield it from the reactants. Also, back reaction can occur towards an equilibrium state. In the latter, i.e., in the distillation zone whereat alkylation is occurring, the products are constantly stripped from the catalyst bed and the benzene is refluxed back into the bed, whereby back reaction is eliminated or at least virtually eliminated. Also, since there is no competition for catalytic sites, the reaction can proceed at a faster pace and/or towards a different product mix. Some catalysts work well in both environments, others in one but not the other. A single catalyst which could be used in either environment would be useful in that a refinery carrying out alkylation of aromatics using both types of alkylation zones would only need to maintain supplies of a single catalyst.
It should also be recognized that such reactions as the conversion of benzene to produce high purity cumene via alkylation are carried out under much different reaction conditions than is the reaction of a refinery stream having only a limited amount of benzene along with numerous other hydrocarbons with olefins to form a low benzene content gasoline blend stock. In the former instance one operates with an excess, often a great excess, of the benzene as compared to the olefin. For example, olefin to benzene mole ratios of 1:4 to 1:30 are common for such reactions. By contrast, the olefin to benzene mole ratio for producing a refinery stream having only a limited amount of benzene from a refinery stream having several percent benzene would generally be above 0.5:1 and might commonly fall within the range from about 0.5:1 to about 5:1 or even 10:1. Thus, the fact that a particular catalyst might be usable and practical for cumene production does not mean that the same catalyst will be useful for reducing the benzene content of a refinery stream.
In one aspect, the present invention overcomes the disadvantages of the prior art in that extremely efficient alkylation of the benzene in a refinery stream is accomplished generally over 60% conversion, and even up to 90% or more conversion, utilizing a specific catalyst and specific process conditions when carrying out the alkylation.